For example, a LCD device includes: a TFT substrate having thin film transistors (TFTs) and pixel electrodes connected thereto are arranged in a matrix pattern; a counter substrate placed so as to face the TFT substrate and having a color filter, a common electrode, etc. formed thereon; and a liquid crystal layer interposed between the TFT substrate and the counter substrate.
A backlight unit as a light source is provided on the opposite side of the TFT substrate from the liquid crystal layer. A glass substrate is typically used as the TFT substrate. It is known to form over the TFT substrate a light-shielding film, which blocks incidence of light of the backlight unit on the TFTs, and a so-called back gate electrode.
For example, as shown in FIG. 22, which is a cross-sectional view of a conventional semiconductor device, a light-shielding film 101 comprised of a so-called refractory metal is formed on a glass substrate 102. The light-shielding film 101 is covered by an insulating film 103. An island-shaped silicon layer 104 is formed on the surface of the insulating film 103 so as to overlap the light-shielding film 101. Moreover, a gate insulating film 105 is formed over the insulating film 103 so as to cover the silicon layer 104, and a gate electrode 106 is formed on the surface of the gate insulting film 105.
In this conventional semiconductor device, since the light-shielding film 101, the silicon layer 104, etc. are sequentially formed from the side of the glass substrate 102, the stepped shape of the light-shielding film 101 is reflected in the silicon layer 104. Accordingly, it is difficult to crystallize the silicon layer 104 with high accuracy by laser. Moreover, since the stepped portions of the light-shielding film 101, grain boundary ridges, etc. adversely affect the flatness of the surface of the silicon layer 104, it is difficult to form a thin gate insulating film 105. Thus, the threshold voltage of TFTs cannot be controlled with high accuracy, which increases the power supply voltage. Accordingly, it is difficult to reduce power consumption of the semiconductor device. Moreover, an increase in leakage current due to crystal defects in the semiconductor layer 104 results in increased power consumption.
As a solution to this problem, a substrate bonding method using monocrystalline silicon has been proposed as described below.
In a manufacturing method proposed in Patent Document 1, as shown in FIGS. 23-25, which are cross-sectional views illustrating a manufacturing process, a plurality of light-shielding films 101 and an insulating film 103 covering the light-shielding films 101 are first formed on a transparent support substrate 102. Next, as shown in FIG. 24, the surface of the insulating film 103 is planarized by a chemical mechanical polishing (CMP) method. Then, a buried oxide film 107 formed at the surface of monocrystalline silicon 108 is bonded to the planarized surface of the insulating film 103. Thereafter, as shown in FIG. 25, the monocrystalline silicon over the support substrate 102 is reduced in thickness, whereby TFTs are manufactured.
In a manufacturing method proposed in Patent Document 2, as shown in FIGS. 26-27, which are cross-sectional views illustrating a manufacturing process, a silicon substrate 108 is formed so as to have an uneven surface, and the uneven surface is covered by an insulating film 109. Then, an opening 110 is formed in the insulating film 109 in a protruding region of the silicon substrate 108. Thereafter, a back gate insulating film 111 and a conductive material layer 112 are formed on the entire surface of the insulating film 109 including the opening 110, and then the conductive material layer 112 is polished. A back gate electrode 113 is formed by the conductive material layer 112 remaining in the opening 110. Subsequently, an interlayer insulating film 114 is formed so as to cover the back gate electrode 113 and the back gate insulating film 111, and the surface of the interlayer insulating film 114 is bonded to a support substrate (not shown).